There are various reasons why a payload may need to be jettisoned from a satellite. Typically, it is a safety technique used in last resort when the payload cannot be recovered on returning to Earth, or when the payload has failed and cannot be repaired, or when the payload constitutes a danger for satellite integrity.
The ideal jettisoning device is one that makes it possible, on actuation, to cut all physical ties between the payload and the satellite instantaneously while being capable of imparting a velocity to the payload such as to enable it to cover a predetermined safety distance in the shortest time. Jettisoning should take place in controlled manner with minimum transfer of energy to the spacecraft so as to reduce perturbations induced by rapid separation of the payload, i.e. by the reaction forces due to the fact that releasing the stored ejection energy requires devices to be activated that cause transient impacts to be exchanged between the various components of a jettisoning device. Such reduction in perturbations is a condition that is very difficult to satisfy in practice and it corresponds to one of the priority needs in the art in question that has yet to find a satisfactory solution.
In this respect, two elements are critical for satisfying this condition: the device that stores the energy for ejecting the payload; and the device that provides controlled release of the energy stored in this way.
Essentially two types of solution are used for the device that stores jettisoning energy:
preloaded springs in which the jettisoning energy is stored in the form of elastic energy; or
devices containing a gas or a liquid under pressure for providing the thrust required for jettisoning.
The solution using fluid under pressure is of complex design and is used particularly for jettisoning large-sized items, such as launcher fairings, rather than payloads having the dimensions under consideration in the present invention.
Devices based on the use of springs are very common: to this end a variety of designs exist with different spring dispositions. It is common to use one or more helical springs that are preloaded in compression. Devices using a plurality of springs appear to satisfy the requirements of the designers of this type of device, particularly because they increase operating safety given that failure or jamming are the most probable types of breakdown for a spring.
Naturally, the disposition of a spring is closely tied to the retaining device adopted for holding the spring under tension, i.e. the latching device, and also to the guide device which is normally provided for controlling the motion of the payload while it is being subjected to the action of a spring.
Pyrotechnical devices have been very widely used for actuating jettisoning devices and for unlatching the retaining devices.
In particular, explosive nuts and bolt-cutters can constitute a satisfactory solution, particularly since they are now standard tools. However, they cannot be used on their own because of the limit on the size of the part to be cut (the part is typically 12.7 mm (0.5 inches) in diameter, which in turn puts a limit on the load-carrying capability of the system). To overcome these limitations, a dedicated latching device could be used for activation by firing a pyrotechnical device; alternatively, it would be possible merely to use a set of pyrotechnical devices.
However, the increasing complexity of the project makes the use of pyrotechnical devices not very attractive. In addition, a synchronization problem may arise in a distributed system of pyrotechnical devices. Solutions to this problem exist, but they involve further penalties in terms of design complexity.
In any event, the operating safety of pyrotechnical devices may be questioned if their performance is considered in terms of lifetime and the principle on which they operate. No matter what the success rate of samples tested, it nevertheless remains possible that a pyrotechnical device may be defective and fail.
Pyrotechnical bolt-cutters are often used for separating a Marman clamp. This type of clamp is doubtless the device that is most commonly in use, probably because of the simplicity of its operating principle.
Nevertheless, it suffers from a certain number of drawbacks which, in the opinion of the Applicant, are usually overlooked by persons skilled in the art. These drawbacks are as follows:
a. The activation dynamics for releasing a pretensioned clamp constitute a stochastic event. The vibrations of the clamp and its interactions with the surrounding items can be predicted only with the help of dedicated computer models, and only to a certain extent. Real dynamic behavior may differ significantly because of strong dependence on certain parameters, such as prestress, interface geometry, and manufacturing tolerances, and it may exhibit peculiarities. This is normally taken into account by adding devices for catching the clamp after it has been released and for damping vibrations. Also, confidence in the repeatability of clamp behavior is based on testing which can be very expensive, in particular for items of large size, and is sensitive to variations in the parameters mentioned above.
b. To ensure that the jettison device opens properly, the clamp must be released extremely quickly. This may be difficult to achieve in practice, and jamming may take place because of asymmetrical spring action, particularly when a plurality of springs are used.
c. To prevent jamming with bolt-cutters, the clamp must be free to rotate without limit. This means that it may also be necessary to have specially designed bolt-clamping devices.
Naturally that increases the complexity of a project that was initially simple. An interesting solution is that adopted for the Cassini spin and jettison device. A Marman clamp is made up of a certain number of individual clamps uniformly spaced apart around a separation ring. Each individual clamp is activated by a preloaded spring and is held in place by a prestressed cable. The cable is cut with pyrotechnical devices. This solution can allow some mass saving, in particular for clamps of large size. A container is naturally also provided for the cable.
Three publications are mentioned below illustrating the state of the art relating to the field of the present invention.
The article "The design and analysis of a double swivel toggle release mechanism for the Orbiter stabilized payload deployment system" by G. King and T. Sai, 23rd Aerospace Mechanisms Symposium, 3-5 May, 1989, pp. 39-57, NASA-CP-3032 describes a release mechanism which uses a swivel screw hinge and three retraction means acting radially for release purposes.
Nevertheless, in that solution, the release device is activated by pyrotechnical means.
The article "A clamp mechanism for deployable three-ton payloads" by R. Birner and H. Ral. 15th Aerospace Mechanisms Symposium, May 14-15 1981, pp. 375-390, NASA-CP-2181 describes, inter alia, a latching or clamping device in which the latching means is withdrawn by means of a system including a hinge and a slide.
Also, the article "A spring-actuated spin and ejection device for interplanetary missions" by V. Comparetto and P. Coste, 4th European Space Mechanism and Tribology Symposium, 20-22 Sep., 1989, pp. 207-214, ESA-SP-299 describes the use of a Marman clamp which includes a plurality of individual clamps held in place by a cable.
An object of the present invention is thus to provide a device for jettisoning a payload which satisfies practical requirements better than prior art devices of the same type and for the same purpose, in particular with respect to:
the energy interchanged between the payload and the satellite is minimized, while nevertheless enabling payload separation to take place in a predefined sequence; this has the consequence of reducing the vibration and perturbations induced on the satellite by jettisoning the payload (which may be a problem in some applications), i.e. on the experiments or operations that are taking place when jettisoning from the satellite occurs, with this being for the purpose of avoiding significant data loss or damage to on-board instruments or even the interruption of all on-going experiments or operations; in this respect, i.e. jettisoning while inducing only small disturbances on the satellite and thus having only a small impact on the orientation thereof, it was decided during development of the jettisoning device of the present invention to eliminate the use of Marman clamps and also the use of pyrotechnical devices as mentioned above;
the initial velocity of the jettisoned payload ensures that given clearance from the satellite is obtained after a predetermined length of time, with said clearance being specified as an allowable safety "envelope" for the payload, so that after separation from the satellite the payload is free to move without interfering therewith;
it has adequate stiffness to enable the payload to operate in orbit and for transmitting launch or re-entry loads, which may be a decisive factor in the design of a jettisoning device, depending on the characteristics of the payload;
there is no risk of failure or jamming in the spring that stores the minimum energy required for imparting the ejection velocity to the payload; and
no guidance is required for the payload.
The objects mentioned above make it possible to associate simple structure with increased lifetime, thereby making the jettisoning device of the invention highly effective, i.e. behaving very similarly to the ideal device mentioned above.